Optical data storage systems provide a means for storing great quantities of data. The data is accessed by focusing a laser beam to a small spot on the data layer of the optical medium and then detecting the reflected light beam. Optical disk drive systems with removable optical disks are the most common form of optical storage. Various kinds of such systems are known. In a ROM (Read Only Memory) system, such as a compact disk system (e.g. CD-ROM, CD-Audio, CD-Video), data is permanently embedded as marks in the disk at the time of manufacture of the disk. The data is detected as a change in reflectivity as the laser beam passes over the data marks. A WORM (Write-Once Read-Many) system allows the user to write data by making marks, such as pits, in a recording layer of the optical disk. Once the data is recorded onto the disk it cannot be erased. The data in a WORM system is also detected as a change in reflectivity. There are also erasable optical data storage systems, such as phase change and magneto-optic (M-O) systems. While phase change systems also read data by sensing a change in reflectivity, M-O systems read data by measuring the rotation of the incident polarization caused by the M-O media.
To increase the storage capacity of an optical disk, multiple data layer systems have been proposed. An optical disk having two or more data layers may be accessed at different layers by changing the focal position of the lens. IBM's U.S. Pat. No. 5,202,875 describes a multiple data layer optical disk drive system wherein the optical disk comprises either a plurality of substrates, each with a data layer spaced apart by air gaps, or a plurality of data layers in a solid structure. U.S. Pat. No. 4,450,553 assigned to U.S. Philips, uses a solid structure with multiple data layers, where each data layer is a CD type data layer. In such multiple data layer optical disk drive systems, the light from the laser must pass through one or more of the light-transmissive disk substrates to access the data layers on the different substrates. The focusing of the light through a relatively thick light-transmissive substrate introduces spherical aberration into the light beam. If not corrected, this prevents the nearly diffraction-limited spot size from being achieved. In the case of conventional single-disk optical disk drives, this can be corrected by slightly changing the shape of the surfaces of the focusing lens by a fixed amount because the amount of substrate material through which the light must pass remains fixed. However, in multiple data layer optical disk drives, because the light is required to be focused through different numbers and thicknesses of substrates depending on which data layer is being accessed, some form of adjustable active compensation for spherical aberration is required. For example, U.S. Pat. No. 5,097,464 assigned to Matsushita, describes a multiple data layer CD system that uses a lens with aberration corrected for the data layer farthest from the lens and an optical path length corrector that is inserted into the optical path when the laser spot is focused on data layers closer to the lens. Also, IBM's U.S. Pat. No. 5,202,875 describes a multiple data layer optical disk drive system with an active aberration compensator.
An additional problem in multiple data layer systems is the need to be compatible with conventional single data layer disks, such as CDs. For example, conventional CDs use a 1.20 mm thick polycarbonate substrate. A multiple data layer CD disk drive must be able to focus a nearly diffraction-limited spot on the data layer of the conventional single data layer disk as well as on the multiple data layers of the newer multiple data layer disk.
Still another problem that occurs in multiple data layer systems is crosstalk caused by undesirable data, focus error and tracking error signals from adjacent data layers interfering with the signals from the desired data layer. These undesired signals are known as interlayer crosstalk. To avoid this problem it is necessary to maintain a relatively large spacing between the data layers. However, a relatively large spacing between data layers that are separated by solid spacer material increases the overall thickness of the disk and the amount of required active spherical aberration correction, both of which are undesirable.
What is needed is an optical disk drive that operates with single and multiple data layer disks and that minimizes the effects of spherical aberration and interlayer crosstalk.